OpenCitations API

Free OpenCitations API with no key: open citation data linking DOIs — who cites whom, with timespans. An open alternative to proprietary citation indexes. Tested.

No API key requiredCORS enabledHTTPSFree tier

Endpoint tested and returned HTTP 200 on 20 Aug 2026

What is the OpenCitations API?

OpenCitations is a free, key-free API providing open citation data — which publications cite which others, identified by DOI, with the timespan between them — as an open alternative to proprietary citation databases.

Citation data has historically been locked inside expensive commercial products like Web of Science and Scopus. OpenCitations publishes it as open data under a CC0 waiver, which is a genuinely significant thing for open scholarship.

The API is DOI-centric and symmetrical: `/references/{doi}` gives what a paper cites, `/citations/{doi}` gives what cites it. Between them you can walk a citation graph in either direction without a subscription.

Quick facts

Base URL: https://api.opencitations.net/index/v1
Authentication: No API key required. A free token raises rate limits for heavy use.
Rate limit: No published hard limit for reasonable use; heavy consumers should request a token.
Pricing: Free. Citation data is released under a CC0 public domain waiver.
CORS: Enabled — callable directly from browser JavaScript
Official docs: Read the docs

How to use the OpenCitations API

Every request below was executed against the live API on 20 Aug 2026, and the response shown is the real body it returned — not an illustration.

1. Fetch the references of a paper

GET https://api.opencitations.net/index/v1/references/10.1186/1756-8722-6-59

curl

curl 'https://api.opencitations.net/index/v1/references/10.1186/1756-8722-6-59'

JavaScript (fetch)

const res = await fetch("https://api.opencitations.net/index/v1/references/10.1186/1756-8722-6-59");
if (!res.ok) throw new Error(`Request failed: ${res.status}`);
const data = await res.json();
console.log(data);

Python (requests)

import requests

res = requests.get("https://api.opencitations.net/index/v1/references/10.1186/1756-8722-6-59", timeout=20)
res.raise_for_status()
print(res.json())

Response — HTTP 200 (truncated)

[
  {
    "oci": "06190834283-06703097368",
    "citing": "10.1186/1756-8722-6-59",
    "cited": "10.1016/s0006-291x(02)02643-8",
    "creation": "2013-08-19",
    "timespan": "P10Y8M",
    "journal_sc": "no",
    "author_sc": "no"
  },
  {
    "oci": "06190834283-062501781527",
    "citing": "10.1186/1756-8722-6-59",
    "cited": "10.1182/blood.v120.21.187.187",
    "creation": "2013-08-19",
    "timespan": "P0Y9M3D",
    "journal_sc": "no",
    "author_sc": "no"
  },
  {
    "oci": "06190834283-0614089124",
    "citing": "10.1186/1756-8722-6-59",
    "cited": "10.1002/ajh.23433",
    "creation": "2013-08-19",
    "timespan": "P0Y4M22D",
    "journal_sc": "no",
    "author_sc": "no"
  },
  {
    "oci": "06190834283-06901809082",
    "citing": "10.1186/1756-8722-6-59",
    "cited": "10.1182/blood-2011-10-386417",
    "creation": "2013-08-19",
    "timespan": "P1Y6M17D",
    "journal_sc": "no",
    "author_sc": "no"
  },
  {
    "oci": "06190834283-0625078255",
    "citing": "10.1186/1756-8722-6-59",
    "cited": "10.1074/jbc.274.14.9587",
    "creation": "2013-08-19",
    "timespan": "P14Y4M",
    "journal_sc": "no",
    "author_sc": "no"
  },
  {
    "oci": "06190834283-062101350134",
    "citing": "10.1186/1756-8722-6-59",
    "cited": "10.1016/0014-5793(94)00783-7",
    "creation": "2013-08-19",
    "timespan": "P18Y11M28D",
    "journal_sc": "no",
    "author_sc": "no"
  },
  {
    "oci": "06190834283-06150171289",
    "citing": "10.1186/1756-8722-6-59",
    "cited": "10.1002/eji.1830231210",
    "creation": "2013-08-19",
    "timespan": "P19Y8M",
    "journal_sc": "no"

Parameters

Parameter	Type	Required	Description
`references/<doi>`	path	Optional	What this DOI cites. `references/10.1186/1756-8722-6-59`
`citations/<doi>`	path	Optional	What cites this DOI. `citations/10.1186/...`
`citation-count/<doi>`	path	Optional	Just the citation count. `citation-count/10.1186/...`
`metadata/<doi>`	path	Optional	Bibliographic metadata for a DOI. `metadata/10.1186/...`

Response fields

ocistring: Open Citation Identifier for this citation link.
citingstring: DOI of the citing publication.
citedstring: DOI of the cited publication.
creationstring: Date the citing work was published.
timespanstring: ISO 8601 duration between cited and citing publication.
journal_sc / author_scstring: Whether it is a journal or author self-citation.

What you can build with the OpenCitations API

Build citation network visualisations without a subscription
Calculate citation counts from open data
Analyse self-citation rates in a field
Support open bibliometrics research

Common errors and how to fix them

Empty array

No citation data indexed for that DOI.

Fix: Coverage is good but not complete — it depends on publishers depositing open references.

Slow responses

Highly cited papers return large arrays.

Fix: Use /citation-count when you only need the number.

Old URL 404s

The service moved to api.opencitations.net.

Fix: Use the api. subdomain; older opencitations.net/index paths redirect.

OpenCitations API — frequently asked questions

Is OpenCitations free?

Yes, free with no API key, and the citation data is released under a CC0 public domain waiver.

Why does open citation data matter?

Citation indexes have historically been proprietary and expensive. OpenCitations makes the underlying who-cites-whom data freely available, which enables bibliometrics research without a subscription.

Is coverage complete?

No. It depends on publishers depositing open reference lists with Crossref. Coverage is substantial and growing but not universal, so counts may be lower than commercial indexes.

How do I just get a citation count?

Use /citation-count/{doi}, which returns the number without the full array — much faster for highly cited papers.

Tools that pair with this API

JSON Formatter

Format, beautify and minify JSON online with 2-space, 4-space or tab indentation. Sort keys alphabetically and catch syntax errors instantly — free and private.

CSV to JSON Converter

Convert CSV to a JSON array of objects online. Header-row detection, comma/semicolon/tab delimiters and pretty-printed output — 100% in your browser.

Alternatives to OpenCitations API

If OpenCitations does not fit, these science & space apis cover similar ground. Every one was called before publishing, and each links to its own reference page with a working curl command and the response it returned.

API	What it does	Auth	CORS	Last verified
NASA APOD	Free NASA Astronomy Picture of the Day API. Daily space images with expert explanations, HD URLs and archive access back to 1995. Tested curl example.	Key	Yes	19 Aug 2026
Open Notify ISS	Free API returning the International Space Station's live latitude and longitude. No key, updates in real time. Tested curl example and live JSON response.	No key	Yes	19 Aug 2026
USGS Earthquake	Free USGS earthquake API with no key: live global seismic events as GeoJSON, filterable by magnitude, time and location. Tested curl example and live response.	No key	Yes	19 Aug 2026
Launch Library 2	Free rocket launch API with no key: upcoming and past launches from every space agency, with rockets, missions, pads and live stream links. Tested example.	No key	Yes	19 Aug 2026
OPSIN	Free OPSIN web service at EMBL-EBI with no key: parse systematic IUPAC chemical names into SMILES, InChI or CML algorithmically. Open source, deterministic, no lookup table.	No key	Yes	21 Aug 2026
ORCID	Free ORCID public API with no key: researcher profiles, publications, affiliations and funding by persistent researcher identifier. Tested curl example.	No key	Yes	20 Aug 2026

All Science & Space APIs

OpenCitations is an independent third-party service and is not affiliated with ByteTools or ByteVancer. Details on this page were verified on 20 Aug 2026; always check the official documentation before relying on this API in production, as terms and limits can change.